Throughout history there has been an on-going quest by engineers to develop cheaper and/or stronger structural members such as beams or girders for all manner of structures including buildings, bridges, ship structures, truck bodies and chassis, aircraft and the like.
For several millennia timber was the primary source of material for structural beams in buildings and bridges and the last several centuries in particular have seen dramatic advancements from timber to cast iron to wrought iron to mild steels and thence to sophisticated steel alloys. Along with the advancement in structural beam materials has gone improvements in fabrication techniques and this, in turn, has permitted significant advances in structural engineering. Throughout this period of change and development in structural engineering, history has witnessed the emergence of unique driving forces which have had a profound influence on the nature and direction of these changes and developments. These drivers have included labour costs, material costs and, of more recent times, environmental issues.
U.S. Design Pat. Nos. 27394 and 28864 illustrate early forms of an I-beam and C-channel respectively while U.S. Pat. No. 426,558 illustrates early forms of hollow flanged beams, possibly made by a casting process.
Improvements in fabrication methods then led to structural members of reduced mass whilst retaining structural performance. U.S. Pat. No. 1,377,251 is indicative of a cold roll forming process of a hollow flanged trough channel, while U.S. Pat. No. 3,199,174 describes a method of fabrication and reinforcement of I-shaped beams by welding together separate strips of metal. U.S. Pat. No. 4,468,946 describes a method for fabrication of a beam having a lambda-shaped cross-section by bending a sheet of metal, and U.S. Pat. No. 4,433,565 describes the manufacture by cold or hot shaping of metal members having a variety of cross-sectional shapes. U.S. Pat. No. 3,860,781 and Russian Inventor's Certificate 245935 both describe the automated fabrication of I-beams from separate web and flange strips fused together. U.S. Pat. No. 5,022,210 describes a milled timber beam having a solid central web portion narrower than solid flanges extending along opposite sides of the web.
Composite beam or truss structures fabricated from a plurality of components are known to provide good strength to weight ratios as illustrated in U.S. Pat. No. 5,012,626 which describes an I-beam-like structure having planar flanges connected to a transversely corrugated web. Other transversely corrugated web beams are disclosed in U.S. Pat. Nos. 3,362,056 and 6,415,577, both of which contemplate hollow flange members of rectangular cross-section. Other transversely corrugated web beams with hollow rectangular cross-section flanges are described in Australian Patent 716272 and Australian Patent Application AU 1986-52906. A method of fabrication of hollow flanged beams with corrugated webs is disclosed in U.S. Pat. No. 4,750,663.
While the prior art is replete with structural members and beams of widely varying configurations, a majority of such structural members or beams have been designed with a specific end use in mind although some are designed as general purpose beams to replace say, a conventional hot rolled I-beam. U.S. Pat. No. 3,241,285 describes a hollow fabricated beam of thin austenitic stainless steel which offers high strength to weight ratios and lower maintenance costs than hot rolled I-beams in bridge building applications. Another type of fabricated bridge girder known as the “Delta” girder is described in AISC Engineering Journal, October 1964, pages 132-136. In this design, one or both of the flange plates is stiffened by bracing plates extending the full length of the beam on both sides between the flange plate(s) and the web.
U.S. Pat. No. 5,692,353 describes a composite beam comprising cold rolled triangular hollow section flanges separated by spaced wooden blocks for use as prefabricated roof and floor trusses. United Kingdom Patent Application GB 2 093 886 describes a cold rolled roofing purlin having a generally J-shaped cross-section, while United Kingdom Patent Application GB 2 102 465 describes an I- or H-section beam rolled from a single strip of metal. International Publication WO 96/23939 describes a C-section purlin for use in a roof supporting building, and U.S. Pat. No. 3,256,670 describes a sheet metal joist having a double thickness web with hollow flanges, the web and the flanges being perforated to allow the joist to be incorporated into a cast concrete floor structure.
U.S. Pat. No. 6,436,552 describes a cold roll formed thin sheet metal structural member having hollow flanges separated by a web member. This member is intended to function as a chord member in a roof truss or floor joist.
The aforementioned examples of structural members or beams represent only a small fraction of the on-going endeavours to provide improvements in beams for a plethora of applications. The present invention however, is specifically concerned with hollow flanged beams of which an early example is described in U.S. Pat. No. 426,558 mentioned earlier herein. The use of hollow flanges to increase the flange section without adding mass is well known in the art. Another early example of hollow flanged beams is described in U.S. Pat. No. 991,603 in which the free edges of triangular cross-section flanges are returned to the web without welding to the web. Similar unwelded hollow flanged beams are described in U.S. Pat. No. 3,342,007 and International Publication WO 91/17328.
Hollow flanged I-beam-like structures, with fillet welded connections between the flanges and the web are described in U.S. Pat. No. 3,517,474 and Russian Inventor's Certificate 827723. An extruded aluminium beam shown in Swedish Publication Number 444464 is formed with a ribbed planar web with hollow rectangular flanges protruding from one web face, the hollow flanges being formed by U-shaped extrusions which clip into spaced receiving ribs formed on one face of the web.
U.S. Pat. No. 3,698,224 discloses the formation of H- and I-beams and a channel section with hollow flanges by deforming welded seam steel tubing to form a double thickness web between spaced hollow flanges.
U.S. Pat. Nos. 6,115,986 and 6,397,550 and Korean Patent Application KR 2001077017 A, describe cold roll formed thin steel structural members having hollow flanges with a lip extending from each flange being secured against the face of the web by spot welds, rivets or clinches. The beams described in U.S. Pat. Nos. 6,115,986 and 6,397,550 are employed as wall studs which enable cladding to be secured to the hollow flanges by screws or nails.
British Patent No GB 2 261 248 describes hollow flanged torsion resistant ladder stiles formed by extrusion or cold roll forming.
U.S. Pat. No. 6,591,576 discloses a hollow flanged channel shaped structural member with a cross-sectionally curved web shaped by press forming to produce a longitudinally arcuate bumper bar reinforcing member for a motor vehicle.
While most of the hollow flanged structural members described above were fabricated with a closed flange with an unfixed free edge or otherwise disclosed a fixed free edge by welding or the like in a separate process, U.S. Pat. No. 5,163,225 described for the first time a cold rolling process wherein free edges of hollow flanges were fixed to the edges of the web in an in-line dual welding process. This beam was known as the “Dogbone” (Registered Trade Mark) beam and possessed hollow flanges of generally triangular cross-section. U.S. Pat. No. 5,373,679 describes a dual welded hollow flange “Dogbone” beam made by the process of U.S. Pat. No. 5,163,225. Such was the performance for price offered by these beams that a low mass thinner sectioned hot rolled universal beam was introduced into the market to counter the perceived threat to conventional universal beams of I- or H-cross-section.
Further developments of the dual weld “Dogbone” process described in U.S. Pat. No. 5,163,225 were disclosed in U.S. Pat. No. 5,403,986 which dealt with the manufacture of hollow flange beams wherein the flange(s) and the web(s) were formed from separate strips of metal as distinct from a single strip of metal in U.S. Pat. No. 5,163,225. A further development of the multiple strip process for forming hollow flange beams was described in U.S. Pat. No. 5,501,053 which taught a hollow flange beam with a slotted aperture extending longitudinally of at least one flange to permit telescopic engagement of a flange of one hollow flange beam within a hollow flange of an adjacent beam for use in structural applications as piling, walling, structural barriers or the like.
A still further development of the dual welding “Dogbone” process is described in Australian Patent 724555 and U.S. Design Pat. No. Des 417290. A hollow flange beam is formed as a channel section to act as upper and lower chords of a truss beam with a fabricated web structure secured in the channelled recess in the chord members.
While generally superior to other hollow flange beams of similar mass, the hollow flange “Dogbone” beams suffered a number of limitations both in manufacture and in performance. In a manufacturing sense, the range of sizes of “Dogbone” beams available from a conventional tube mill was limited at a lower end by the proximity of inner mill rolls and otherwise limited at a larger end by the size of the roll stands. While “Dogbone” beams generally exhibited increased capacity per unit mass or per unit cost when compared to conventional “open” (unwelded) hollow flange beams or conventional angle sections, I-beams, H-beams and channels, they also exhibited a surprisingly high torsional rigidity and thus a resistance to flexural (lateral) torsional buckling over longer lengths. These hollow flange beams failed due to a unique lateral distortional buckling mode of failure not found in other similar products. Similarly, while the sloping inner flange faces provided an excellent deterrent for avian and rodent pests in some structural applications, the capacity for the flange to resist local bearing failure was less than other beams such as I-beams due to flange crushing. Additionally, special attachment fittings were required because of the cross-sectional shape.
Conventionally, the selection of a structural beam for use in a structure was usually made by an engineer after reference to standard engineering tables to ascertain section efficiencies and load bearing capacity in a range of readily available “standard” beams such as laminated timber, hot rolled H-, L- or I-beams and channels, cold rolled beams such as C-, Z-, J-shaped purlins or the like. The higher the value of bending capacity per unit mass, the more efficient the section. This value measures the performance per unit cost thus allowing a comparison of cost efficiencies of various beams by taking into account the cost per unit mass for each product.
Where special performance requirements are demanded of a beam, cost or cost efficiency may be governed by other factors and often this is the impetus to design a special purpose beam for a specific application. Otherwise, as the prior art so clearly demonstrates, there has been and there continues to be an on-going quest to produce more cost effective general purpose beams having greater section efficiencies than widely used conventional general purpose timber laminate beams, hot rolled I-, L- and H-beams, hot rolled channels and cold rolled purlin beams of various cross-sectional shapes. The fact that few, if any of the plethora of prior art “improvements” has been adopted for widespread use is probably due to an inability to combine both general cost efficiency with general section efficiency.
The assignee of the present invention, is successor in title to the “Dogbone” dual weld hollow flange beam inventions and has conducted an exhaustive survey into actual costs of incorporating a “Dogbone”-type beam into a structure with a view to designing a hollow flange dual welded cold rolled general purpose beam which, between manufacture, handling and transportation and ultimate incorporation in a structure, was more cost effective in a holistic sense than any of the prior art conventional general purpose beams which otherwise overcame several recognized disadvantages in the “Dogbone” beam, namely, connectivity and a capacity for flange crushing with localized loads.
A conjoint research methodology was developed to measure the individual product attribute utility for various beam profiles with builders, engineers and architects. These key attributes were then assigned values to produce a utility rating from which a customer value analysis for various types of beams could enable a direct comparison based on many product attributes other than merely cost/unit mass and section efficiency. From this customer value utility analysis, a range of dual welded hollow flange beam configurations in both mild steel and thin gauge high strength steel were devised as potential replacements for hot rolled steel beams such as I- and H-beams and hot rolled channel as well as laminated timber beams.
Among the many attributes considered in relation to hot rolled steel beams, connectivity and cost of handling with cranes were significant issues. U.S. Pat. No. 6,637,172, which describes a clip to enable attachment to the flanges of hot rolled structural beams, is indicative of the connectivity problems of such beams. As far as timber was concerned, dwindling availability, length availability, termites, straightness, and weather deterioration were significant factors which adversely affected customer value analyses.
Accordingly, it is an aim of the present invention to overcome or alleviate at least some of the disadvantages of prior art general purpose structural beams and to provide a structural beam of greater overall customer utility than such prior art general purpose structure beams.